Batteries of all types, whether frequently or rarely used, discharge with the passage of time. Accordingly, when placed into service, the battery or batteries that provide the energy source for electronic equipment such as flashlights, toys, radios, audiocassette players, compact disc players and myriad other devices, may or may not have sufficient charge to power the equipment. As a consequence, a variety of devices have been proposed whereby a user can determine a battery's energy level with reasonable certainty.
Perhaps the earliest known battery testers, which remain in common usage, include specifically designated voltmeters and ammeters. Although quite accurate when properly calibrated and operated, such devices are cumbersome to use, must be carefully maintained and stored, and can be rather expensive.
More recently, battery testers have been incorporated into battery packaging containers. These testers, generally referred to as thermochromic testers, normally compromise an electrically conductive layer in thermal contact with a temperature sensitive color indicator layer. When the ends of the conductive layer are contacted with a battery's terminals, electronic current flows through and creates heat in the conductive layer. The heat so generated causes a change in the indicator layer if the voltage of the battery exceeds a predetermined threshold. Tester devices of this sort are somewhat difficult to operate because a user must precisely align and maintain contact of the tester's terminals with the battery's terminals to achieve reliable results. In addition, the tester is usually capable of testing the condition of only a single size of battery, e.g., a AA battery. Moreover, the battery packaging itself, which is bulky and susceptible to damage, must be retained and carefully stored, although it is commonly misplaced or discarded as trash.
Even more recently, thermochromic testers have been incorporated into the labels encasing the batteries themselves. Examples of such built-in testers may be found in U.S. Pat. No. 5,015,544. Testers of this sort are in immediate contact with the typically metal housings of the batteries to which they are attached. As such, the battery may act as a heat sink for the heat generated during operation of the tester. If not controlled this loss of heat may hinder the function of both the tester and the battery. For instance, to achieve the threshold temperature sufficient to effect a change in the color indicator layer, the tester may have to be operated for a longer period of time than would otherwise be desired, thereby prematurely draining the battery of useful energy. Additionally, the loss of tester heat into the battery may cause the tester to produce inaccurate readings of the battery's strength, i.e., the tester might indicate the battery to be drained when in fact the battery is still useful. In these circumstances, a user might mistakenly discard good batteries in reliance upon the errant readings of the tester.
To alleviate battery heat sink problems, U.S. Pat. Nos. 5,059,895, 5,223,003, 5,389,458, 5,393,618, 5,409,788 and 5,538,806 have proposed placement of thermal insulation means between the conductive layer of a thermochromic battery tester label and the battery housing. The thermal insulation means may comprise, inter alia, a layer of release paper, plastic strips, foamed plastic, foamed ink, embossed or printed inks, adhesives, cloth and the like. These insulation materials may be deployed as substantially continuous layers or as discontinuous arrangements defining one or more air pockets between the conductive layer and the battery housing. Indeed, because of the extremely low thermal conductivity of air, when the insulating material is discontinuous in layout, the insulating characteristics of the composite thermal insulation means, i.e., insulation material and air gaps, are superior to a continuous layer of insulating material having no air gaps. U.S. Pat. No. 5,223,003 in particular discloses thermochromic battery tester thermal insulation means including solid spacers formed of foamed plastic and shaped to define an air pocket. It will be appreciated that such a construction combines the thermal insulation benefits of the air pocket, the plastic material and air contained in the plastic material.
When cured, foamed plastics or inks comprise a matrix of plastic or ink material which entraps bubbles of a gaseous matter, most commonly air. It is axiomatic that there is a direct relationship between the volume of entrapped gas and the thermal insulation characteristics of the insulation material: the greater the volume of entrapped gas, the more thermally insulative the material, and vice versa. However, there is an inverse relationship between entrapped gas volume and the structural strength of the insulating material. That is, as the "hollow" or "void" space of a foamed plastic increases, the material's ability to resist externally applied compressive force decreases, and vice versa. Hence, the volume of entrapped gas cannot exceed a threshold level which would compromise the insulating structure's capacity to withstand externally imposed forces associated with ordinary manufacturing shipping, handling and usage of a typical battery. Yet, this threshold entrapped gas level may not be sufficient to impart a meaningful contribution to the thermal insulation characteristics of the material.
A need exists, therefore, for a thin thermal insulation means for a thermochromic battery tester that combines optimum thermal insulation properties with high structural strength and that will function effectively with any battery of conventional dimensions.